Yes surely some days are cloudy
So some days you get 5% capacity factor, and need some other energy source as well
So it harms the economics of the venture
Look at the profitability of companies building utility scale solar farms, they cost 100 million and the company hopes to get a 10% return and pay a 3% dividend.
They still have to contend with moving parts for tracking the angle of the sun, fans on inverters, contactors, clearing snow, mowing grass, site drainage, tornadoes etc, so sometimes it is not as easy as it sounds
All for a 7%? Why shouldn’t they just buy the s&p 500 and call it a day
But to be clear, it's less about night vs day and more about summer vs winter.
I had a 20kWh array and 18kWh of batteries in Texas and it was GREAT in the summer. It'd start charging by 6am and be charged by 9am, even with simultaneous usage. Then we'd live off solar for the day (even with HVAC), go back on batteries around 9pm and they'd be out around 4am. No problem.
But during an overcast winter day, the stack wouldn't get power until 8/9, not make it to 50%, start discharging by 4/5pm, and be out by 10/11pm. It would easily be 8-10 hours where we were wholly dependent on the grid.
Not a problem, just a constraint to acknowledge and plan for.